Cathodes are known to be an important factor in reducing cell performance in fuel cells. The cathode reaction in fuel cells using a proton conductor as the electrolyte is broken down into the three following processes.    (1): oxygen decomposition activation 1/2O2+2e−→O2−    (2): migration of oxygen ions O2− ((1) reaction field)    (3): production of water 2H−+O2−→H2O
The (1) the oxygen decomposition activation is assumed to be the rate-limiting process, and attempts have been made in the past to expand the reaction field to increase the activity in (1).
For example, the use of the noble metal Pt for the cathode will increase the reactivity in (1), but Pt lacks any oxygen ion transport capacity, thus limiting the reaction field of (1) to the proximity of the triple-phase boundary where the reaction of (3) takes place. It is thus impossible to obtain high activity in terms of the cathode as a whole. Large amounts of expensive noble metal must also be used.
By contrast, a technique has been proposed for using a complex oxide (a combined oxygen ion/electron conductor) having both oxygen ion and electron conductivity, such as La0.6Sr0.4CoO3 (LSC), as the cathode (see, for example, Patent Citation 1). The oxygen ion conductivity of the complex oxide allows the O2− that is produced to be used in the water-producing reaction over the entire surface of the cathode. When the LSC does not have enough O2− decomposition activity, platinum can be supported on the surface to enhance the reactivity in (1).
However, even though the above combined oxygen ion/electron conductor is endowed with sufficient oxygen ion conductivity and electron conductivity at temperatures of 800 degrees or higher, a problem is the insufficient oxygen ion conductivity and electron conductivity in the intermediate temperature range of 200 to 600 degrees, for example. Oxygen ion conductivity, in particular, is a temperature-related function. Oxygen ion conductivity decreases as the temperature drops.
Cathodes comprising combined oxygen ion/electron conductors thus cannot be used in fuel cells with an operating temperature range in the intermediate temperature range of 200 to 600 degrees, and the power-generating performance of fuel cells cannot be improved.